1. Field of the Invention
This invention relates to an electronic endoscope comprising a flexible conduit or scope and a video-signal processing unit to which the flexible scope is detachably connected at a proximal end.
2. Description of the Related Art
In such an electronic endoscope, the flexible conduit or scope includes an objective lens system provided at the distal end thereof, and a solid image sensor, such as a CCD (charge-coupled-device) image sensor, associated therewith. The flexible scope also includes an optical light guide extended therethrough, formed as a bundle of optical fibers, which is associated with a lighting lens system provided at the distal end of the flexible scope.
On the other hand, the video-signal processing unit includes a white-light source, such as a halogen lamp, a xenon lamp or the like. When the flexible scope is connected to the video-signal processing unit, the proximal end of the optical light guide is optically connected to the light source. Thus, an object to be photographed is illuminated by light radiating from the distal end of the optical light guide, and is focused as an optical image on a light-receiving surface of the CCD image sensor by the objective lens system.
The focused optical image is converted into a frame of analog image-pixel signals by the CCD image sensor. Then, the frame of analog image-pixel signals is read from the CCD image sensor, and is fed to the video-signal processing unit, in which the image-pixel signals are suitably processed, thereby producing a video signal including image-pixel signals and various synchronizing signals. Then, the video signal is fed from the video-signal processing unit to a TV monitor to reproduce the photographed object on the monitor on the basis of the video signal.
In general, the objective lens system, used in the electronic endoscope, exhibits a large depth of focus, because a close-range object image and/or a distant-range object image to be photographed must be focused on the light-receiving surface of the CCD image sensor by the objective lens system, before the photographed close-range object image and/or distant-range object image can be sharply reproduced on the monitor.
In this case, to maintain a constant overall brightness of a reproduced object image on the monitor, the radiation of the illuminating-light from the distal end of the optical light guide should be regulated in accordance with a distance between the photographed object image and the distal end of the optical light guide. For example, when only a medical image of a patient is to be reproduced as a close-up image by placing the distal end of the flexible scope close to the medical image, the radiation of the illuminating-light should be lowered to a minimum level in order to generate the medical image at a predetermined brightness on the monitor. Then, as the distal end of the flexible scope is moved away from the medical image, the radiation of the illuminating-light should be gradually increased from the minimum level to prevent the brightness of the reproduced medical image from being reduced.
Conventionally, for a regulation of radiation of the illuminating-light from the distal end of the optical light guide, an aperture-stop is associated with the white-light source, and is automatically controlled such that an overall brightness of the reproduced object image is always maintained at a constant level. In particular, a frame of luminance signals is extracted from the video signal at given regular time-intervals, and an average luminance level is calculated from the extracted luminance signals. Then, the radiation of the illuminating-light from the distal end of the optical light guide is regulated by controlling the aperture-stop such that the average luminance level coincides with a predetermined reference level.
Nevertheless, in the conventional electronic endoscope, there is room for improvement in a responsiveness of the regulation of the radiation of the illuminating-light from the distal end of the optical light guide, because a calculation time for calculating the average luminance level is relatively long, and because the calculation must be repeated at very short-time intervals. For example, in an electronic endoscope using the NTSC system, the calculation must be repeated at regular time-intervals of {fraction (1/30)} sec, and, in an electronic endoscope using the PAL system, the calculation must be repeated at regular time-intervals of {fraction (1/25)} sec.
On the other hand, in general, an electronic endoscope is constituted such that a photographed image is reproduced as a color image. In this case, a frame of red image-pixel signals, a frame of green image-pixel and a frame of blue image-pixel signals are cyclically read out from the CCD image sensor, and are then subjected to a white-balance correction processing such that the photographed color image is reproduced on a monitor with a proper color balance. As is well-known, the white-balance correction processing is performed by processing respective gains of red, green and blue image-pixel signals with red, green and blue correction factors, which exhibit inherent values with respect to each individual CCD image sensor used in an electronic endoscope. Thus, the correction factors are determined during manufacture of the electronic endoscope.
In particular, a manufactured flexible scope concerned is connected to a so-called master video-signal processor, and a distal end of the flexible scope is inserted into a tubular-like envelope, an inner wall surface of which is coated with a standard white pigment layer. Then, a frame of red image-pixel signals, a frame green image-pixel signals and a frame of blue image-pixel signals are obtained from the CCD image sensor, and a red correction factor, a green correction factor and a blue correction factor are determined on the basis of the obtained color image-pixel signals such that gains of red, green and blue image-pixel signals are equal to each other.
Nevertheless, an electronic characteristic of the master video-signal processor does not necessarily coincide with that of a manufactured and used video-signal processor to which the flexible endoscope concerned is connected. Thus, the determined correction factors are not necessarily proper with respect to the manufactured and used video-signal processor. Further, the color correction factors should be periodically readjusted and redetermined in accordance with deterioration of a white-light lamp which harmfully affects the white-balance correction processing. Namely, there is a demand for an improved electronic endoscope in which the redetermination of the color correction factors can be easily carried out.
Therefore, an object of the present invention is to provide an electronic endoscope in which a responsiveness of a regulation of radiation of an illuminating-light from a distal end of an optical light guide can be favorably improved.
Another object of the present invention is to provide an electronic endoscope in which it is possible to easily readjust and redetermine color correction factors for a white-balance correction.
In accordance with the present invention, there is provided an electronic endoscope which comprises a flexible scope, and a video-signal processing unit to which a proximal end of the flexible scope is detachably connected. The flexible scope has an image sensor provided at a distal end of the scope, and an optical light guide extending through the scope. The video-signal processing unit processes image-pixel signals successively read from the image sensor, and is provided with a light source such that light, emitted from the light source, is guided through the optical light guide and radiates from the distal end of the flexible scope. The electronic end scope features a light-emission regulator which may be an aperture-stop associated with the light source, and the aperture-stop regulates the radiation of light from the distal end of the flexible scope. The electronic endoscope further features a histogram generator which successively generates a luminance-signal-histogram in accordance with the processed image-pixel signals, an approximate-calculator which approximately calculates an approximate average luminance level-value in accordance with luminance signals exhibiting thinned luminance levels extracted from the luminance-signal-histogram, and a controller which controls the light-emission regulator in accordance with the approximate average luminance level-value, such that an image having a constant brightness level is reproduced in accordance with the processed image-pixel signals.
The generation of the luminance-signal-histogram by the histogram generator may be based on either a frame or field of image-pixel signals extracted from the processed image-pixel signals. The thinned luminance levels may be obtained by suitably extracting a series of luminance levels from the luminance-signal-histogram at regular intervals. Preferably, the thinned luminance levels are alternately extracted from the luminance-signal-histogram.
The approximate calculation of the average luminance level-value by the calculator may be performed over a restricted range of a full histogram-definition range of the luminance-signal-histogram, and the restricted range may be defined in accordance with another luminance-signal-histogram generated in the histogram-generator prior to the generation of the luminance-signal-histogram. Preferably, the restricted range is obtained by marginally extending a histogram range defined by a minimum luminance level and a maximum luminance level of another luminance-signal-histogram generated in the histogram-generator prior to the generation of the luminance-signal-histogram.
Preferably, the electronic endoscope further comprises an exact-calculator, which periodically and exactly calculates an exact average luminance level-value in accordance with luminance signals included in the luminance-signal-histogram, and the controller periodically controls the light-emission regulator in accordance with the exact average luminance level-value.
The electronic endoscope may be constituted such that a gain-correction factor adjustment mode or white-balance-readjustment mode is selected as an operation mode of the endoscope. In this case, the image sensor successively is constituted so as to generates a first frame of monochromatic image-pixel signals, a second frame of monochromatic image-pixel signals and a third frame of monochromatic image-pixel signals; the flexible scope further has a memory that stores a first gain-correction factor, a second gain-correction factor and a third gain-correction factor; the video-signal processing unit is constituted so as to read the first, second and third gain-correction factors from the memory when the flexible scope is connected to the unit, and so as to process the first, second and third frames of monochromatic image-pixel signals with the first, second and third gain-correction factors, respectively; and the histogram generator is constituted so as to successively generate a first image-pixel-signal-histogram, a second image-pixel-signal-histogram and a third image-pixel-signal-histogram in accordance with the first, second and third frames of monochromatic image-pixel signals, processed with the first gain-correction factor, the second gain-correction factor and the third correction factor, respectively. The electronic endoscope further comprises: a first calculator that calculates a first average signal-level-value in accordance with image-pixel signals included in the first image-pixel-signal-histogram; a second calculator that calculates a second average signal-level-value in accordance with of image-pixel signals included in the second image-pixel-signal-histogram; a third calculator that calculates a third average signal-level-value in accordance with of image-pixel signals included in the third image-pixel-signal-histogram; and a gain-correction-factor adjuster that adjusts at least two of the first, second and third gain-correction factors such that the first, second and third average signal-level-values are substantially equal to each other.
Preferably, the electronic endoscope comprises a writer which writes the adjusted gain-correction factors in the memory of the flexible scope. Also, the electronic endoscope preferably comprises an operation-mode selector an operation-mode selector that switches an operation mode of the endoscope from a usual-operation mode to the gain-correction factor adjustment mode, and the adjustment of the gain-correction factors by the gain-correction-factor adjuster is performed when the gain-correction factor adjustment mode or white-balance-readjustment mode is selected by the operation-mode selector.